This invention relates to a rotary flowmeter used to measure the flow of a fluid and, more particularly, to a flowmeter that has a moveable needle that moves within a channel having a varying area.
There are of course, various means of measuring and indicating the flow of a fluid. One such means involves a circular guage having a moveable needle. The flow to be measured is directed towards one end of the moveable needle through an orifice and the fluid impinging against the needle causes it to rotate. A scale is provided that radially surrounds the needle and the needle position can be read from the scale in units of flow. Typical of such flowmeters is shown in Gauthier, U.S. Pat. No. 2,268,391. Actual movement of the needle is caused by drag force on the needle and which tends to move the needle in the direction of flow of the fluid. A return spring is provided to return the needle to its zero position when the flow of fluid has ceased and to provide a counter force opposite the drag force exerted on the needle by the flow of fluid.
A difficulty with such flowmeters relates to their principle of operation. The velocity of the fluid that exits the orifice to impinge on the moveable needle is generally linear with increasing flow, however, the drag force that moves the needle increases with the square of the velocity of the fluid that impinges on that needle. Thus, as the velocity of the fluid impinging on the needle increases, indicating increasing flow, the needle moves angularly further for the same incremental increase in flow rate, and, as a result the scale is generally expanded at the high flows and compressed at the lower flows. That is, the scale indicia are not linearly arranged radially around the needle since needle movement becomes more progressed per unit flow as that flow increases.
There are many applications where an expanded scale at higher flows is undesirable since in many instances, for the total scale movement, greater accuracy may be desired at lower flows, therefore greater scale spacing is advantageous at the lower flows. Alternatively, there are instances where the overall scale is desired to be linear so that each increasing increment of flow would result in the same incremental angular movement of the needle no matter whether the overall flow is low or high.